1. Field of the Invention
The present invention relates to a communication device, typically a receiver, that conducts wireless communication by, for example, DS-CDMA (direct sequence-code division multiple access) using an adaptive array antenna, and more particularly, to a technology for improving calibration accuracy at the time of calibrating communication antenna chains and for improving the reception quality of received signals.
2. Description of the Prior Art
In recent years, consideration has been given to CDMA transceiver circuits that utilize adaptive array antennas (AAAs).
When an adaptive array antenna is used to receive and transmit signals, its multiple antenna elements are individually weighted to control the directivity of the overall antenna. At the time of signal reception, for example, the adaptive array antenna is controlled so as to achieve maximum directivity in the direction of the desired incoming wave and to strongly depress received signal quality with respect to signals from other directions. This type of control operation is conducted by using a control algorithm which is provided in the communication device.
FIG. 5 shows calculated antenna directivity patterns of an adaptive array antenna. Specifically, the pattern designated (a) is an example in which the maximum directivity has been adjusted to 0 degrees for the reception of an incoming wave from the 0-degree direction, and the pattern designated (b) is an example in which the maximum directivity as been adjusted to 45 degrees for the reception of an incoming wave from the 45-degree direction. For reference, FIG. 5 also shows the directivity component in the 180-direction that is the opposite direction from 0 degrees, and the −45-degree direction that is the opposite direction from 45 degrees.
When received signal processing is conducted by using an adaptive array antenna to control directivity in this manner, the received signal processing can be conducted while eliminating interference waves arriving from directions which are different from the arrival direction of the desired signal. The adaptive array antenna has therefore drawn considerable attention as a technology for eliminating interference.
The foregoing explanation regarding directivity during reception also applies to the directivity and interference removal effect during transmission, except that special measurement is required.
The special measurement that is required for an adaptive array antenna during transmission will now be explained.
This special measurement is called “calibration.” Calibration is for correcting phase and amplitude deviation that arises in transmission and reception chains including antennas and in the individual devices making up the chains owing to variations occurring during manufacture.
This will be explained with reference to a specific problem. Assume, for example, that based on phase that is calculated from receiver output varying in phase and amplitude (gain), a mobile station is detected in the direction of 45 degrees as viewed from a base station. At transmission, the directivity of the adaptive array antenna should be adjusted to be maximum in the direction of 45 degrees. If the transmitter section has not been calibrated, however, it is actually impossible to impart high antenna directivity in the 45-degree direction because phase differences and level differences (amplitude differences) are present in the individual antenna chains. Moreover, if the receiver section has not been calibrated, the detection of the mobile station in the 45-degree direction cannot be relied on from the start. The end result is that the transmit directivity will be adjusted to a direction which is different from the direction where the mobile station is located.
Types of communication antenna chain calibration include, for example, receive calibration for correcting phase and amplitude deviation between antenna chains when receiving signals using communication antenna chains, transmit calibration for correcting phase and amplitude between antenna chains when transmitting signals using a communication antenna chains, and receive/transmit calibration for correcting phase/amplitude deviation with respect to a single communication antenna chain between the case where the communication antenna chain is used to receive signals and the case where the communication antenna chain is used to transmit signals.
The importance of communication antenna chain calibration is obvious from the attention it has received in, for instance, “Indoor transmission characteristics of adaptive antenna-array transmission diversity in W-CDMA downlink, Harada et al., Technical Report of The Institute of Electronics, Information and Communication Engineers, RCS99-18 (1999-05),” “Study on RF transmission and reception circuit calibration in W-CDMA downlink adaptive antenna-array transmission diversity, Harada, Tanaka, Sawabashi and Adachi, Technical Report of The Institute of Electronics, Information and Communication Engineers, RCS99-101 (1999-08)” (hereinafter “Reference 1”), and “Automatic calibration method for FDD system adaptive array that takes antenna characteristics into account,” Nishimori, Osa, Takatori and Hori, Technical Report of The Institute of Electronics, Information and Communication Engineers, RCS99-213, MW99-233 (2000-02) (hereinafter “Reference 2”).
Reference 1 reports, among other matters, that the relative amplitude/phase deviation between RF receiver units and between RF transmitter units is substantially constant irrespective of signal transmission power and reception power.
Reference 2 reports, among other matters, that since a DBF (digital beam forming) configuration is generally adopted to use the baseband for adaptive array antenna directivity control, calibration of amplitude ratio and phase differences between the antenna branches arising in the RF section etc. and of the differences between receive chain and transmission chain is required to realize ideal transmission and reception patterns, that device amplitude/phase characteristics vary over time with changes in environment and temperature, and that in FDD (frequency division duplex), the fact that the transmit frequency and receive frequency differ necessitates the calibration of the antenna cable (such antenna cable calibration being actually required when installing the equipment).
A CDMA base station equipped with both an adaptive array antenna and a calibration information analyzer will now be explained.
FIG. 6 is a block diagram showing one configuration of such a CDMA base station. The CDMA base station is equipped with n number of communication (transmission/reception) antennas L1-Ln constituting an adaptive array antenna, n number of transceiver units (TRX) M1-Mn, and n number of calibration information CAL information) analyzers N1-Nn. The antennas L1-Ln, transceiver units M1-Mn and calibration information analyzers N1-Nn are combined in sets so as to form n number of communication antenna chains. The CDMA base station is also equipped with a user-segregated AAA signal processor and discriminator 78 which is common to the n number of communication antenna chains. The symbol n designates an integer greater than 1.
The transceiver units M1-Mn are all the same in configuration and operation. The transceiver unit M1, for instance, comprises a receiver unit including a mixer 71, an attenuator 72, a directional coupler 73, a frequency converter 74 and an orthogonal detector 75. The structural particulars of the transceiver units M1-Mn are not illustrated and will not be explained here.
The calibration information analyzers N1-Nn are all the same in configuration and operation. The CAL information analyzer N1, for instance, includes a despreader 76 and a phase/gain error detector 77.
The antennas L1-Ln receive and transmit spread spectrum signals from and to mobile stations (users).
The transceiver units M1-Mn carry out receive and transmit processing. In receive processing, the frequency converters 74 frequency-convert (down-convert) the received signals which are received from the antennas L1-Ln from the carrier frequency band (radio frequency (RF) band in this example) to an intermediate frequency (IF) band, and the orthogonal detectors 75 effect orthogonal detection to the baseband frequency band and output the orthogonal detection results to the calibration information analyzers N1-Nn.
In transmit processing, orthogonal modulators (not shown) orthogonally modulate transmit signals which are received from the calibration information analyzers N1-Nn from the baseband frequency band to the intermediate frequency band, and frequency converters (not shown) frequency-convert (up-convert) the results of the orthogonal modulation to the carrier frequency band and output the results to the antennas L1-Ln.
The calibration information analyzers N1-Nn detect phase and amplitude deviations that are caused by the transmission and reception chains and devices, and carry out calibration based on the detection results.
The user-segregated AAA signal processor and discriminator 78 conducts transmit and receive processing. In receive processing, the user-segregated AAA signal processor and discriminator 78 acquires the received signals of the communication antenna chains from the calibration information analyzers N1-Nn, multiplies these signals and receive weights (weighting coefficients) that are applied to the communication antenna chains and synthesizes the multiplication results for all communication antenna chains so as to produce a synthesis result that is a received signal of the adaptive array antenna, where this processing is done for every mobile station. The user-segregated AAA signal processor and discriminator 78 then demodulates the received signals, discriminates their data, and outputs the so-obtained user-specific data (individual user data) for X number of users. X can be singular or plural. X is equal to, for example, the number of mobile stations from which the CDMA base station is simultaneously receiving signals.
In transmit processing, the user-segregated AAA signal processor and discriminator 78 modulates user-specific data for X′ number of users, sums the modulated signals to produce transmit signals, multiplies the transmit signals and transmit weights that are applied to the communication antenna chains, and outputs the multiplication results for the respective communication antenna chains to the transceiver units M1-Mn via the calibration information analyzers N1-Nn. X′ can be singular or plural. X′ is equal to, for example, the number of mobile stations to which the CDMA base station simultaneously transmits signals.
The mixer 71, attenuator 72, directional coupler 73, despreader 76 and phase/gain error detector 77 are circuit components provided for calibration. These circuit components do not operate when calibration is not an objective.
The operation of the transceiver unit M1 and the CAL information analyzer N1 will be explained by way of example.
The communication antenna L1 receives a signal that is transmitted by a mobile station (a signal that is a basic object of the reception; hereinafter sometimes called “object signal”) and signals that are transmitted by other communication antennas L2-Ln (other communication antenna transmit signals).
The mixer 71 produces a signal of a frequency which is equal to the frequency difference (transmit/receive difference frequency) between the signal of frequency f1 that is transmitted from the CDMA base station to a mobile station and the signal of frequency f2 of the signal that is transmitted from the mobile station to the CDMA base station (i.e., the signal the CDMA base station receives from the mobile station), converts the frequency f1 of the other communication antenna transmit signals which are contained in the received signal to the frequency f2 of the object signal, and outputs the converted other communication antenna signals to the attenuator 72.
The attenuator 72 attenuates the other communication antenna transmit signals that are received from the mixer 71 and outputs the attenuated other communication antenna transmit signals to the directional coupler 73. The power of the communication antenna transmit signals is reduced by attenuation because the power of signals which are transmitted from the CDMA base station to the mobile station is much greater than the power of signals which are received by the CDMA base station from mobile stations. That is, the power of the communication antenna transmit signals is attenuated in order to avoid the great difference in power that would arise if the other communication antenna transmit signals should be mixed with the object signal received without reducing their power level.
The directional coupler 73 receives the other communication antenna signals from the attenuator 72, mixes them with the incoming received signal from the communication antenna L1, and outputs the mixed signal to the frequency converter 74.
The mixed signal that is output by the directional coupler 73 is processed as explained above by the frequency converter 74 and the orthogonal detector 75, and is output to the CAL information analyzer N1.
The despreader 76 receives the received signal from the transceiver unit M1, uses spreading codes which are associated with the other communication antenna transmit signals that are contained in the received signal so as to despread (demodulate) the other communication antenna transmit signals, and outputs the result of the despreading to the phase/gain error detector 77.
Based on the despread result received from the despreader 76, the phase/gain error detector 77 detects the phase deviations and gain deviations of the other communication antenna transmit signals that are indicated by the despread result.
The foregoing procedure enables the detection of the mutual phase and gain deviations (errors) among the antennas L1-Ln. When, for example, the communication antenna L1 is used exclusively for receiving and the signals that are transmitted from the communication antenna L2 and the communication antenna L3 are received by the communication antenna L1 and their deviations are detected, the deviations between the communication antenna L2 and the communication antenna L3 can be acquired.
However, in a conventional CDMA base station that conducts calibration, such as that shown in FIG. 6, the fact that transmission is conducted at high power during system operation makes it difficult to secure an adequate dynamic range in the calibration circuits which are provided in the transceiver units M1-Mn, particularly in the mixer 71. Moreover, the conventional CDMA base station also has other problems, such as that the attenuator 72 has to be controlled according to the system state and that the transceiver units M1-Mn are structurally complex.
Moreover, during operation of the conventional CDMA base station, high-power signals from the other communication antennas get mixed in with the received signal. This degrades the reception quality of the object signal from the mobile station (hereinafter called “Problem A”).
One conceivable method of coping with Problem A would be, for example, to divide the transmit signal that is transmitted from the other communication antennas, send the divided signals to the communication antenna that receives the transmit signal, and use the deviation which is obtained from the phase/gain error detector 77 of the communication antenna chain concerned and the divided signals so as to remove the other communication antenna transmit signals from the received signal in the calibration information analyzer N1. However, as mentioned above, since the large transmission power occurring during system operation makes it difficult to secure an adequate dynamic range, detection of the exact value and removal would become extremely difficult during operation. Moreover, since the number of transceiver units depends on the n number of antennas making up the adaptive array antenna, the transceiver unit configuration would become complex and greatly increase equipment cost. Furthermore, the transceiver units cannot readily be miniaturized or decreased in cost by large scale integration (LSI) or like technologies because they are analog units.
The present invention was made in light of the foregoing shortcomings of the prior art. One object of the invention is to provide a communication device which enables enhanced calibration accuracy and improved reception quality of communication signals when calibration is effected during the transmission and reception of signals using a communication antenna. Another object of the invention is to provide a communication device that eliminates the need for an attenuator (such as the attenuator 72), enables the detection of calibration information using a low-power signal, achieves simplification of the transceiver unit structure, and enables reduction of the equipment size and cost.